Traditionally, DC power on board aircraft has been derived from three-phase AC power provided by on-board generators, using a Transformer Rectifier Unit (TRU) or an Autotransformer Rectifier Unit (ATRU). Aspects of exemplary prior art systems are described in, for example, U.S. Pat. Nos. 4,866,591, 4,739,446, 6,256,213, 7,796,413, and 8,687,394. The output of the on-board generators, which are driven by the aircraft engines, is typically 115 VAC three phase power at nominally 400 Hz. However, the frequency may vary widely depending on engine conditions (e.g., in the range of 300-800 Hz). The TRU converts this unregulated frequency, three phase voltage into regulated DC power to supply various on board systems.
The traditional prior art TRU employs a relatively large line frequency transformer (nominally operating at 400 Hz) to provide galvanic isolation. Typically, the transformer creates additional phases from the unregulated three phase AC input. In such traditional TRU, the multiple phases appearing at the output of the transformer are each provided to a set of silicon rectifiers that rectify the power to provide a DC output voltage, e.g., 28V DC.
To some extent, the harmonic currents created by the rectifiers may be cancelled in the transformer by synthesis of additional phases in the transformer windings. For example, if three additional phases are synthesized by the transformer windings for a total of six phases, (three original phases plus three more), then for each line cycle, the output diodes will conduct 12 times, resulting in a 12-pulse TRU. (Other pulse count TRUs may be constructed by synthesizing different numbers of additional phases).
Typically, the higher the pulse count, the more harmonics will be cancelled to provide a current waveform that more closely resembles a sine wave. However, these types of traditional TRU units lack good regulation, and are not effective at rejecting transient voltages that may appear at their inputs.
To address this problem, some aerospace power system suppliers have replaced the output rectifiers with silicon-controlled rectifiers (SCRs) and incorporate designs that control the conduction phase angle of the SCRs to regulate the DC output voltage. Such units are commonly referred to as Regulating Transformer Rectifier Units (R-TRUs).
However, traditional TRU and R-TRU units suffer from multiple drawbacks. For example, they require a relatively large 400 Hz line frequency transformer that, in an aircraft environment, takes up valuable space and increases weight. They also exhibit relatively poor energy conversion efficiency due to the high voltage drop that occurs in their output rectifiers. For example, prior art R-TRU designs typically do not provide more than approximately 85% efficiency.
Of particular importance to aircraft power distribution systems is protection against damaging failure modes. For example, an over-voltage at the DC output has the potential of creating a hazardous condition on the aircraft, by either damaging critical downstream load equipment so that the equipment no longer functions, damaging redundant equipment due to an over-voltage condition that crosses between redundant power busses, or causing smoke and fire due to damage to equipment that can lead to failure of critical aircraft systems. In a worst case scenario, such damage can result in catastrophic failure of the aircraft itself.
Traditional R-TRUs may be subject to various failure modes that can cause a dangerous over-voltage condition. Prior art systems incorporating traditional R-TRUs therefore must include auxiliary safety circuits that prevent over-voltage, leading to additional expense and size of such systems. Accordingly, there is a need in this field for more efficiency power systems that provide good regulation of the DC output, and that also provide built-in safeguards to prevent over-voltage conditions from damaging the aircraft or downstream systems.
To overcome the failings of prior art designs, the disclosed system utilizes a high frequency switching power conversion architecture that provides a regulated DC power system that is small in size, highly efficient in terms of energy conversion, and that incorporates multiple layers of protection against damaging over-voltage conditions.
Efficiencies using the circuit topologies disclosed herein can be 30% above prior art designs, e.g., in the approximate range of 96-98%, and may provide DC power systems capable of outputting 7-10 KW or more, typically providing 28 V DC output voltages and capable of providing 100-400 A current outputs.
In accordance with one aspect of the present disclosure, an R-TRU having the foregoing advantages of small size and high efficiency is provided to convert a three phase AC input voltage into a regulated DC output voltage, by utilizing a Vienna Rectifier have power factor correction circuitry for actively increasing the power factor towards unity.
In accordance with another aspect of the present disclosure the Vienna Rectifier is coupled to a DC to DC converter, which in a preferred embodiment is a series resonant DC to DC converter whose output may be controlled using phase shift modulation control over its duty cycle.
In accordance with yet another aspect of the present disclosure, one or more compensator circuits are provided in feedback arrangements that are coupled to the Vienna Rectifier and/or the DC to DC converter to control their respective outputs and prevent a damaging over-voltage condition from appearing at the regulated DC output.